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Cellular Technologies Mobile Device Investigations Program Technical Operations Division - DFB DHS - FLETC Basic Network Design Frequency Reuse and Planning 1. Cellular Technology enables mobile communication because they use of a complex two-way radio system between the mobile unit and the wireless network. 2. It uses radio frequencies (radio channels) over and over again throughout a market with minimal interference, to serve a large number of simultaneous conversations. 3. This concept is the central tenet to cellular design and is called frequency reuse. Basic Network Design Frequency Reuse and Planning 1. Repeatedly reusing radio frequencies over a geographical area. 2. Most frequency reuse plans are produced in groups of seven cells. Basic Network Design Note: Common frequencies are never contiguous 7 7 The U.S. Border Patrol uses a similar scheme with Mobile Radio Frequencies along the Southern border. By alternating frequencies between sectors, all USBP offices can communicate on just two frequencies Basic Network Design Frequency Reuse and Planning 1. There are numerous seven cell frequency reuse groups in each cellular carriers Metropolitan Statistical Area (MSA) or Rural Service Areas (RSA). 2. Higher traffic cells will receive more radio channels according to customer usage or subscriber density. Basic Network Design Frequency Reuse and Planning A frequency reuse plan is defined as how radio frequency (RF) engineers subdivide and assign the FCC allocated radio spectrum throughout the carriers market. Basic Network Design How Frequency Reuse Systems Work In concept frequency reuse maximizes coverage area and simultaneous conversation handling Cellular communication is made possible by the transmission of RF. This is achieved by the use of a powerful antenna broadcasting the signals. Basic Network Design How Frequency Reuse Systems Work If you have one powerful antenna you would be able to handle conversations based on the number of channels you have available. The first step necessary for frequency reuse is the cell system of the network. Basic Network Design How Frequency Reuse Systems Work Each antenna in the various cells operate on the same RF that are assigned. If your original antenna could handle 7 calls and you increased the number of antennas by 10 - 70 simultaneous calls. The concept goes beyond the number of antennas and deals with how the radio frequency itself is used and reused. Basic Network Design How Frequency Reuse Systems Work 1. Each cell has its own antenna and low power Base Station to handle the traffic within it’s area. 2. Each Base Station is assigned frequencies with neighboring base Stations being assigned different frequencies. 3. Carriers are then able to reassign these frequencies to other areas that are not too geographically close to the other Base Stations and cells. Basic Network Design Consider your local radio station broadcast. As you travel father away from the radio base it weakens until you lose the signal. Now consider all the cars driving around you are listening are listening to different stations. The radio station would be the Base Station and the vehicles would be the cells. Basic Network Design Cell Geographical Area (b)(7)e Geographical Limits http://www.soi.wide.ad.jp/class/20000002/slides/06/19.html Basic Network Design Distance to Reuse Ratio 1. The Distance to Reuse ratio defines how much geographical distance is required between cells in a cell system to avoid and limit interference. 2. The overall geographic size of cell base stations along with the power of the antenna determine the distance to reuse ratio Basic Network Design Distance to Reuse Ratio 1. What happens when I move about a cell coverage area or move into another cell area? 2. Another requirement of the wireless system is frequency agility. 3. This is the ability of the mobile unit to operate on any given frequency within their assigned spectrum. 4. This allows the mobile unit to switch from one channel to the another seamlessly and allows for another a important component of cellular technology – Call Hand Off Basic Network Design Call Handoff 1. Call handoff can best be described as the process of passing from one Base Transceiver Station (BTS) to another while maintaining connection to your network. 2. When you leave your particular network and move into another network - the roaming process. This is transparent to the user. Basic Network Design Define Call Handoff 1. Without Call Handoff, Frequency Reuse would not be possible and vice versa. 2. The Mobile Switch Center (MSC) monitors the power levels of the mobile units. 3. When the MSC detects the mobile unit power levels degrading it seeks out other BTSs. Basic Network Design Define Call Handoff Base station coverage overlap with other cells in the area and it is this overlapping that allows call handoff to occur. (b)(7)e http://www.cell-phone101.info/_imagez/passed-signal.gif Basic Network Design 1. Base station coverage overlap with other cells in the area and it is this overlapping that allows call handoff to occur. 2. This action is handled by a microprocessor at the MSC and is seamless to the user. 3. This is a complex act that uses frequency synthesizers, the controller, and memory functions within the wireless handset. 4. This is why cellular phones must have frequency agility or the ability to change from one channel to another. Basic Network Design Wireless Network Design Maps The FCC requires the use of United States Geographical Survey maps in the planning and development of the wireless network. As it relates to planning it is important that all carriers use the same maps to lend conformity to the planning process. Since the BTS operates at specific power levels it is very important to know where other towers are to avoid and/or minimize interference. Basic Network Design Hexagon Grid The hexagon grid design is the predominant engineering design tool in the wireless industry. There are other design models configured in squares, circles, octagons, etc. depending on the number of cell towers in the design. The hexagon is used because it best represents and simulates the seven tower overlapping of circles. Circles being the way radio frequencies are depicted. Basic Network Design Hexagon Grid 2 3 7 1 4 6 5 Basic Network Design In viewing our hexagon grid we can see where frequencies overlap from one cell to the other. (b)(7)e Basic Network Design Hexagon Grid 1. 2. (b)(7)e 3. 4. Basic Network Design Fundamental Components of a Wireless System There are five main components to a wireless network. They are: 1. The Mobile Unit 2. The Cell Base Station 3. The Backhaul or Fixed Network 4. The Mobile Switching Center 5. The interconnection to the Public Switched Telephone Network (PSTN) Page 25 redacted for the following reason: - - - - - - - - - - - - - - - - - - - - - (b)(7)e Basic Network Design There are two classifications of mobiles units in use today when we speak of cellular telephones and mobile devices. The Mobile Unit 1. The Portable telephone or device – these are your small handsets, portable devices with network connection capabilities such as PDA’s and GPS units. 2. The Mobile telephone or device – devices that are mounted in the locomotion device, such as installed telephones and GPS units. Basic Network Design The Cell Base Station 1. The Cell Base Station is the physical location of some of the equipment needed to operate the wireless network, such as antennas, GPS timing systems, cell towers etc. 2. The size of the base station is dependent upon it’s location and system needs. Basic Network Design The Cell Base Station 1. Raw Land Sites 2. Rooftop Sites 3. Water Tank Sites 4. Co-located Sites 5. Stealth Sites Basic Network Design The Cell Base Station 1. Microcells – a outdoors network base station usually on rooftops, water tanks and the like. The Base Station range of a Microcell is generally 100 meters to 1000 meters. 2. Picocells – the smallest, usually used indoors and intended to provide coverage for a small area. The Base Station range of a Picocell is generally less than 100 meters. Typically found in airports, e.g. 3. Nanocells – mobile and easily installed. Nanocells can be mounted on walls, in vehicles or outdoor weatherproof enclosure. Coverage is dependant you configuration. Basic Network Design The Cell Base Station - Macrocells Basic Network Design The Cell Base Station - Microcells Basic Network Design The Cell Base Station - Picocells Basic Network Design The Backhaul or Fixed Network – Base Station Controller 1. A complex collection of systems that connect the Base Stations to the Base Station Controller. 2. This network connects the mobile users to others on its network and to the outside world. 3. THE BSCs are the brains behind the BTS. They usually control numerous cell sites. Basic Network Design The Backhaul or Fixed Network – Base Station Controller 1. The BSC handles the allocation of radio channels, measurements (Power Levels) from mobile devices, and in most cases handoffs from one BTS to another. 2. The BSC concentrates activity to and from the Mobile Switch Center to help with efficiency. 3. It provides data to the Operations Support System to monitor and enhance performance as well as assisting in trouble shooting. 4. The BSC then connects to the Mobile Switching Center. Basic Network Design The Mobile Switching Center 1. Often called the brains of a wireless network, the MSC is responsible for switching data packets from one network path to another. 2. This process is called call routing. 3. MSC provides subscriber service information such as user registration, authentication and location updating. Basic Network Design 1. The MSC provides connection to the Public Switched Telephone Network (PSTN) and the Integrated Services Digital Network (ISDN) using SS7 based interconnection.
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